Solid conductor thermal feedthrough

ABSTRACT

A thermally conductive feedthrough has a conductive member extending through a fiber-reinforced plastic plate. The feedthrough is sealed against leakage from one side of the plate to the other by placing the plate in local compression to seal it against the plate and/or by using small individual conductive members that minimize the effects of thermal expansion differences. The feedthrough can be used between vacuum and cryogenic liquids.

This is a division, of application Ser. No. 08/080,743, filed Jun. 21,1993, now U.S. Pat. No. 5,441,107.

BACKGROUND OF THE INVENTION

This invention relates to a feedthrough across a solid wall, and, moreparticularly, to a thermal feedthrough for a solid conductor.

A feedthrough is a structure that permits the selective transmission ofa flow of energy or mass through a solid wall. Various types of devicesmust operate in isolation from temperatures and particular environments.Such devices are normally placed within a sealed enclosure for theiroperation. However, it often is necessary to selectively transfer sometypes of energy or mass across the walls of the enclosure, at the sametime that the walls prevent the transmission of other types of energy ormass.

As an example, some types of electronic devices must be operated at verylow, cryogenic temperatures in a vacuum. The devices are placed into asealed, insulated enclosure that insulates against a heat flow into theenclosure and thence to the devices. The interior of the insulatedenclosure is evacuated by a vacuum pump either continuously, orinitially and then sealed. With such an insulated enclosure, it isusually necessary to provide for two types of feedthroughs in the wallof the insulated enclosure. One is an electrical signal feedthrough, sothat electronic signals and sometimes power can be transmitted into andout of the enclosure. Various types of electrical feedthroughs are wellknown in the art.

The other type of feedthrough is a thermal feedthrough. A thermalfeedthrough permits a flow of heat to be removed from within theevacuated space interior to the insulated enclosure to an exteriorcooling device, to keep the devices within the insulated enclosurecooled to their operating temperatures. In one possible type of thermalfeedthrough, a solid conductor extends from the interior of theinsulated enclosure to the exterior, and must pass through the wall ofthe enclosure.

Such solid conductor thermal feedthroughs are more difficult toconstruct than electrical feedthroughs, particularly if a vacuum sealmust be maintained across the wall and the feedthrough. The problemarises from the fact that the various components of the wall and thefeedthrough structures generally have different thermal expansioncoefficients. The solid conductor is a metal of high thermalconductivity such as copper, aluminum, or silver. The wall is typicallya nonmetallic structure such as fiberglass-reinforced plastic compositematerial, or a metal structure of low thermal conductivity.

When the interior of the insulated enclosure is cooled, the variouscomponents contract at different rates. The components tend to separatefrom each other as a result of the differing contractions, resulting invacuum leaks across the feedthrough. The problem aggravated if theinsulated enclosure and the feedthrough are repeatedly cooled andheated, as often occurs during cycles of operation, because someportions of the damage induced by the thermal expansion differences canaccumulate with increasing numbers of cycles. The result is anaccumulation of thermal fatigue damage to the structure and its eventualfailure.

There is a need for an improved approach to solid conductor thermalfeedthroughs, which permit efficient heat flow but are resistant todamage such as vacuum leaks induced by single or multiple thermalexcursions. The present invention fulfills this need, and furtherprovides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a thermal feedthrough across a wall for asolid thermal conductor. Heat from one side of the wall is conducted tothe other side of the wall via the solid conductor. The thermalfeedthrough allows this heat to be transferred across the wall, and alsoretains a vacuum-tight hermetic seal at the wall. The vacuum-tight sealis maintained during and after thermal excursions such as the cooling ofthe interior to a cryogenic temperature, either once or many timesduring multiple cycles of operation. The feedthrough may be made in anonmagnetic form, an important attribute for some types of applications.

In accordance with the invention, a thermal feedthrough comprises afiber-reinforced plastic plate having a first surface and a secondsurface, and further having a plate bore therethrough, with the platebore having a first diameter over a first portion of its length and asecond, larger diameter over a second portion of its length adjacent tothe second surface. The b ore of the plate is filled with a metallicconductive plug having a first diameter over a first portion of itslength and a second, larger diameter over a second portion of itslength. There is an adhesive seal between the plate and the plug. Thefirst portion of the plate bore is sufficiently large to receive thefirst portion of the plug therein with an interference fit, and thesecond portion of the plate bore being sufficiently large to receive thesecond portion of the plug therein. Upon assembly, the plate is placedin radial compression in the region of the plate bore.

The method of manufacture of this thermal feedthrough contributessignificantly to its success by introducing the radial compression intothe plate. In accordance with this aspect of the invention, a method ofpreparing a thermal feedthrough comprises the steps of providing afiber-reinforced plastic plate having a first surface and a secondsurface, and further having a plate bore therethrough. The plate borehas a first diameter over a first portion of its length and a second,larger diameter over a second portion of its length adjacent to thesecond surface. The method further includes providing a metallicconductive plug having a first diameter over a first portion of itslength and a second, larger diameter over a second portion of itslength. The first portion of the plate bore is sufficiently large toreceive the first portion of the plug therein with an interference fit,and the second portion of the plate bore is sufficiently large toreceive the second portion of the plug therein with an interference fit.At least one of the plate bore and the plug is coated with an adhesive.The first portion of the plug is forced into the first portion of theplate bore until the second portion of the plug enters the secondportion of the plate bore and bottoms. Additional axial pressure isapplied to the plug after the plug has bottomed. The axial pressure issufficiently high to place the region of the plate adjacent the secondbore into residual compression.

In another aspect of the invention, a thermal feedthrough comprises afiber-reinforced plastic plate having a first surface and a secondsurface, and further having a threaded plate bore therethrough. Athreaded bolt made of a metallic alloy with relatively poor thermalconductivity is engaged to the threaded plate bore. The bolt has aninterior bolt bore therethrough. A first metallic thermal conductorextends through the interior of the interior bolt bore and is sealed tothe internal surface of the bolt bore, as by soldering. A layer of afirst adhesive is disposed between the threads of the bolt and the boreof the plate. A first retainer is engaged between the bolt and the plateadjacent to the first surface of the plate, and a second retainer isengaged between the bolt and the plate adjacent to the second surface ofthe plate. The second retainer includes a volume of a second adhesivecontacting the second surface of the plate, and a nut threadably engagedto the bolt. The first retainer, the second retainer, and the boltcooperate to place the bolt in tension and the plate in compression. Asecond metallic thermal conductor is affixed to the first metallicconductor at a first end thereof, and a third metallic thermal conductoris affixed to the first metallic conductor at a second end thereof.

In yet another embodiment of the invention, a thermal feedthroughcomprises a fiber-reinforced plastic plate having a first surface and asecond surface, and further having a plate bore therethrough. A plug issized to fit within the plate bore and is affixed into the plate bore.The plug comprises a length of a cured fiber-reinforced compositematerial wound onto a cylindrical nonmetallic form into a generallycylindrical, multiturn, jelly roll coil, with the cylindrical axis ofthe coil generally perpendicular to the surfaces of the plate. At leastone thermally conductive wire penetrates between the turns of the coiland through the length of the cylindrical coil generally parallel to acylindrical axis of the coil.

The feedthrough of the invention permits thermal energy to becontrollably introduced into or removed from one side of an enclosurewall with a solid conductor, while maintaining a hermetic, vacuum-tightseal at the wall. The seal is maintained during thermal excursions onone or both sides of the wall, to cryogenic temperatures. Thefeedthrough may be made entirely of nonmagnetic materials, so that itdoes not adversely affect magnetic measurements made in the vicinity ofthe feedthrough. Other features and advantages of the present inventionwill be apparent from the following more detailed description of thepreferred embodiment, taken in conjunction with the accompanyingdrawings, which illustrate, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional view of a structure utilizing a solid thermalconductor and a conductor feedthrough;

FIG. 2 is a side sectional view of a first embodiment of the feedthroughof the invention at various stages of its assembly, showing in FIG. 2(a)the components prior to assembly, in FIG. 2(b) the partially assembledfeedthrough, and in FIG. 2(c) the completed feedthrough;

FIG. 3 is a process flow diagram for a method of preparing the firstembodiment of the feedthrough;

FIG. 4 is a side sectional view of a second embodiment of thefeedthrough of the invention; and

FIG. 5 is a schematic combined side sectional and perspective view of athird embodiment of the feedthrough of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a typical situation where the feedthrough of the presentinvention can be used, although the use of the present invention is notso limited. FIG. 1 illustrates an insulated enclosure 20. The inside ofthe insulated enclosure 20 is evacuated, so that a one-atmospherepressure differential exists across the walls of the insulated enclosure20. A sensor 22, having a pickup coil 22a and a detector 22b connectedby an electrical lead 23, is positioned in the interior of the insulatedenclosure 20. An internal electrical lead 24 extends from the detector22b to an electrical feedthrough 26 of conventional design, placed inthe wall of the insulated enclosure 20. Electrical feedthroughs 26 areavailable commercially from suppliers such as Amphenol or Cannon. Anexternal electrical lead 28 extends from the electrical feedthrough 26to external electronics, not shown.

Separately, an internal solid thermal conductor 30 extends from thesensor 22 to a reservoir 31 located within the vacuum enclosure 20.There is a thermal feedthrough 32 through the wall of the reservoir 31,and a extension solid thermal conductor 34 of the thermal conductor 30within the interior of the reservoir 31. The reservoir 31 contains asupply of a cryogenic liquid. The reservoir is supported from the wallof the insulated enclosure 20 by hollow tubes 36, which vent theinterior of the reservoir 31 and also act as fill tubes for adding thecryogenic liquid to the reservoir. The structure of the thermalfeedthrough 32 is the subject of the present invention, and will bediscussed in greater detail subsequently. The solid thermal conductors30 and 34 may be formed of single metallic conductor or multiplemetallic conductor elements such as braided wires. Substantially purecopper, copper alloys, substantially pure aluminum, aluminum alloys,substantially pure silver, silver alloys, substantially pure gold, andgold alloys are the preferred materials of construction of the thermalconductors 30 and 34.

FIG. 2 depicts a first embodiment of the feedthrough of the invention,and FIG. 3 illustrates the assembly of the feedthrough. Referring inparticular to FIG. 2(a) , a feedthrough 40 includes a plate 42, whichmay be made of a nonmagnetic material, and preferably of afiber-reinforced plastic material such as fiberglass. The plate 42 has afirst surface 44 and a second surface 46. The size of the plate 42 isnot critical, provided that the plate has sufficient strength that itdoes not deform significantly under the one-atmosphere pressuredifferential. By way of illustration and not of limitation, a preferredplate 42 is about 4 centimeters thick and about 43 centimeters indiameter.

A cylindrically symmetric bore 48 extends through the thickness of theplate 42 from the first surface 44 to the second surface 46. The bore 48has two portions along its length, a first portion 50 adjacent to thefirst surface 44 and a second portion 52 adjacent to the second surface46. The first portion 50 has a first diameter and the second portion 52has a second, larger diameter. A shoulder 54 lies between the firstportion 50 and the second portion 52.

In a preferred embodiment, a reentrant recess 56 is positioned aroundthe diameter of the second portion 52 of the bore, at a location wherethe second portion 52 contacts the shoulder 54. The recess 56 is in theform of a toroidal cutout portion or notch extending from the diameterof the second portion 52 to a diameter somewhat greater than thediameter of the first portion 50. By way of illustration and notlimitation, in a preferred embodiment, the first portion 50 has a lengthof 2.84 centimeters and a diameter of 0.95 centimeters, and the secondportion 52 has a length of 0.47 centimeters and a diameter of 1.58centimeters. The recess 56 has a length of 0.15 centimeters.

A cylindrically symmetric plug 58 is sized to fit within the bore 48 ofthe plate 42. The plug is preferably made of substantially pure copper,a copper alloy, substantially pure aluminum, an aluminum alloy,substantially pure silver, a silver alloy, substantially pure gold, or agold alloy. These metals all have acceptable thermal conductivity, withthe pure metals being preferred and pure copper being most preferred.

The plug 58 has a first portion 60 with a maximum diameter sized toachieve an interference fit with the first portion 50 of the bore of theplate 42. The first portion 60 of the plug 58 may have a smooth outerdiameter, or may have a stepped outer diameter, as shown. The steppedouter diameter configuration is preferred, as it aids in achieving agood seal of the plug 58 to the plate 42 and also eases the assemblyoperation. The plug 58 has a second portion 62 sized to achieve a slipfit with the second portion 52 of the bore of the plate 42. Theinterference fit is typically achieved by sizing the outer diameter ofthe first portion 60 of the plug 58 to be about 0.05 millimeters largerthan the inner diameter of the respective portion of the plate 42,within available machining tolerances. Even though the plug is ofslightly larger diameter than the bore, the assembly is achieved byforce fitting the plug into the bore because the plug is made of aslightly compliant material.

The second portion 62 of the plug 58 has a lip 64 extending therefromparallel to the cylindrical axis of the plug 58. The lip 64 isconfigured and sized to fit within the reentrant recess 56 of the secondportion 52 of the bore 48 of the plate 42, with a gap of about 0.05millimeters to allow excess adhesive to be expelled during assembly.

FIGS. 2(b) and 2(c) show the physical relationships of the componentsduring the stages of assembly. Referring to FIGS. 2(b) and 3, toassemble the feedthrough 40, the plate 42 is provided, numeral 70, andthe plug 58 is provided, numeral 72. Immediately before assembly, thefirst and second portions of the bore 48, and/or the first and secondportions of the plug 58 are coated with an adhesive 66, numeral 74. Theadhesive 66 is preferably a curable adhesive such as an epoxy. Anacceptable epoxy is a polyurethane adhesive such as Model 810, made byCrest. This epoxy cures at ambient temperature in a time of about 4 daysafter application, permitting the mechanical assembly to be completedbefore the epoxy hardens. The plug 58 is inserted into the bore 48 andforced downwardly against the interference fit using a tool 67 that fitsagainst the end of the plug 58, numeral 76.

At full insertion, the plug 58 bottoms against the shoulder 54 and thelip 64 engages the reentrant recess 56. At this point, the compressiveforce on the tool 67 is increased to at least about 6000 pounds for atleast about 30 seconds, numeral 78. This compressive force on the tool67 causes the material in area 62 of plug 58 to flow radially outwardlyinto the plate 42, in a region 68 adjacent to the first portion 52 ofthe bore 48. The compressive force is great enough that a 0.25-0.38millimeter impression is left in the plug after the compression tool isremoved. A residual radially inwardly directed compressive force remainsin the region 68, as indicated by the arrows 69 in FIG. 2(c).

Finally, the thermal conductors 30 and 34 are affixed to the oppositeends of the plug 58, numeral 80. The preferred approach to attaching thethermal conductors 30 and 32 is clamped connections using screws orbolts. Alternatively, the conductors can be hard soldered prior toassembly, as long as they are configured so that there is room to usethe tool 67.

In the most demanding type of application, a vacuum is drawn on one sideof the plate 42 (e.g., the interior of the insulated enclosure 20 ofFIG. 1), and the other side of the plate 42 is at atmospheric pressure(e.g., the interior of the reservoir 31). The close fit between the plug58 and the bore 48 of the plate 42, the presence of the epoxy adhesive66, and the radially inward compressive force 69 all cooperate toestablish a vacuum-tight, hermetic seal so that gas cannot leak throughthe feedthrough 40 from the external environment into the interior ofthe vessel. In service, the external thermal conductor 34 is cooled tocryogenic temperature by contact with a heat sink. Heat flows from thesensor 22 along the internal thermal conductor 30, through the plug 58,along the external thermal conductor 34, and to the heat sink. The plug58 and the adjacent portions of the plate 42 are cooled to cryogenictemperatures. The metallic plug 58 has a smaller thermal expansioncoefficient than the fiber-reinforced plastic plate 42. In the coolingprocess, the plug 58 has a natural tendency to contract radially lessthan the plate 42 at the bore 48. It is important to cool the assemblyof plug and plate slowly to prevent the plug from pulling away from theplate. The epoxy adhesive has some compliance and so continues to act asa sealant between the plug and the plate, opposing the tendency for aleak path to open between the plug 58 and the bore 48, so that there isa tendency for a leak path to open between the plug 58 and the bore 48.The epoxy adhesive has some compliancy to prevent such a leak. Theradial relaxation of the residual compressive force 69 in the plate 42also serves to maintain the bore 48 in close contact with the plug 58,also resisting the tendency to form a leak path.

Ten feedthroughs 40 were prepared in a single plate by the approach justdescribed. The plate and feedthroughs were cycled between ambienttemperature and a temperature of 4K for a total of 12 cycles to test thestructure. There were no failures.

A second embodiment of the feedthrough is shown in FIG. 4. A feedthrough90 includes a fiber-reinforced plastic plate 92, which is preferably ofthe same material as the plate 42 described previously. The plate has afirst surface 94 and a second surface 96. The plate 92 has a bore 98therethrough extending between the surfaces 94 and 96. The bore 98 is ofsubstantially constant diameter and is internally threaded.

A bolt 100 is externally threaded with threads to engage the threads ofthe bore 98. The bolt 100 is made of a metallic material such as acopper-beryllium alloy, most preferably an alloy of copper and about 2weight percent beryllium. This alloy has a lower thermal conductivitythan pure copper, but has higher strength. The higher strength isbeneficial in sustaining the axial mechanical loadings present in thebolt 100 that are not imposed upon the plug 58 in the embodiment of FIG.2.

The bolt 100 is of sufficient length to extend between the surfaces 94and 96, and a short distance beyond on each side. The bolt 100 has aninterior bolt bore 102 of a first diameter extending through theinterior of the bolt 100, of sufficient length to extend along most ofthe length of the bolt 100, and a second bore 103 of a small diameterthan the first diameter extending along the balance of the length of thebolt 103. The bolt bores 102 and 103 reduce the effective radial andlongitudinal thermal expansion forces of the bolt 100 when thefeedthrough 90 is cooled during service, aiding in the avoidance of aleak path through the feedthrough 90. A metallic conductor 33 of highthermal conductivity is sealed into the bolt at the bolt bore 103,preferably by hard soldering.

An adhesive layer 104 is present between the bolt 100 and the plate bore98. The adhesive is preferably the same type of epoxy used as theadhesive 66.

A first retainer, preferably a nut 106, is threaded to the end of thebolt 100 extending out the plate 92 from its second surface 96. A nylonwasher 108 is preferably placed between the nut 106 and the secondsurface 95 to seal the adhesive during assembly.

A dam 110 made of a compliant material such as polytetrafluoroethylene(also known as teflon) is placed against the first surface 94. The dam110 has an axial bore 112 that receives the bolt 100 therethrough. Thedam 110 further has an internal cavity 114 that is filled with aflowable adhesive 116 during assembly. The flowable adhesive 116 ispreferably the same material as the adhesive 104 and the adhesive 66.The dam 110 seals against the first surface 94 of the plate 92 andagainst the bolt 100.

A second retainer, preferably a nut 118, is threaded to the end of thebolt 100 extending out of the plate 92 from the first surface 94 and outof the dam 110. A nylon washer 120 is preferably placed between the nut118 and the surface of the dam 110.

One of the metallic thermal conductors 30 is affixed to one end of theconductor 33, and the other of the metallic thermal conductors 34 isaffixed to the other end of the conductor 33. The conductors arepreferably affixed by hard soldering, such as silver soldering at joints35, preferably before the feedthrough is assembled to the plate.

When the feedthrough 90 is assembled, the threads of the bolt 100 and/orthe interior of the plate bore 98 are coated with the flowable,as-yet-uncured epoxy adhesive 104. The bolt 100 is threaded into theplate bore 98, the washer 108 is placed over the end of the bolt 100,and the nut 106 is threaded onto the bolt 100. At the other end, thecavity 114 of the dam 110 is filled with the flowable epoxy adhesive 116and the dam 110 is placed over the end of the bolt 100. The washer 120is placed over the end of the bolt 100, and the nut 118 is threaded ontothe bolt 100.

The nut 106 is tightened slightly to seal the dam 110 to the firstsurface 94 and to the end of the bolt 100. The nut 106 is furthertightened, and the nut 118 may also be tightened. The tightening of thenuts forces flowable epoxy adhesive 116 from the cavity 114 into anyremaining space between the threads of the bolt 100 and the threadedplate bore 98. The state of compression is maintained during the curingof the epoxy adhesive. The dam 110 compresses and deforms. As a resultof this process, any air pockets in the epoxy adhesive are eitherremoved or compressed substantially during curing. The result is avacuum-tight, hermetic seal so that gas cannot leak from one side of theplate 92 to the other during service, even after cooling and heating ofthe feedthrough 90.

The thermal conductors 30 and 34 are preferably attached to the ends ofthe bolt 100 before assembly. In the preferred approach, the conductors30 and 34 are affixed before assembly by silver soldering.

The feedthrough 90 has been constructed in the manner discussed andtested. Two such feedthroughs were cycled 20 times between ambienttemperature and 4K without failure.

The embodiments of FIGS. 2 and 4 are most suited to the situationwherein the thermal conductors 30, 33, and 34 are single metallicpieces. In another construction, the conductors 30 and 34 may be formedas an array of braided or bundled smaller strands. For example, in theone approach the conductors 30 and 34 can be single metallic pieces of adiameter 0.25 centimeters. In the other approach, the conductors 30 and34 can be about 1-100, preferably about 20, metallic strands each of adiameter of 0.3 millimeters.

The embodiment of FIG. 5 is particularly useful for the case where thethermal conductors 30 and 34 are formed as a number of individualconductors, either bundled or braided. The smaller size of theindividual wires lessens the effects of strain caused by differentialthermal contraction upon cooling to cryogenic temperature. A feedthrough130 includes a fiber-reinforced plastic plate 132, preferably made ofthe same material as the plates 42 and 92. The plate 132 has a firstsurface 134 and a second surface 136. A bore 138 extends through theplate 132 from the first surface 134 to the second surface 136.

A plug 140 is formed as a roll 142 of individual turns 144 of a curedand hardened prepreg material. The plug is formed and then machined asnecessary to fit within the bore 138 of the plate 132, and affixedwithin the bore 138 with an adhesive 145. The continuous thermalconductor 30, 34 is made of a plurality of individual strands 146. Theindividual metallic strands 146 are interleafed between the turns 144 ofthe roll 142. Since the individual strands 146 are quite small in size,the difference in absolute dimensional changes induced by thermalexpansion differences between the plate and the plug is quite small. Thecured and hardened prepreg material has some inherent compliancy thatcan accommodate this small absolute dimensional change. Statedalternatively, the thermal expansion dimensional changes which canotherwise lead to leaks through the feedthrough after temperaturechanges during service are spatially diffused sufficiently that thestructure accommodates the changes and no leaks occur.

The plug 140 is manufactured by placing a strip of partially cured(i.e., B-stage cured) fiberglass prepreg material onto a surface. Theprepreg material is soft and can be formed in this state. A cylindricalform 148 is placed at one end, and the strands 146 are placed atpositions along the length of the strip crossing the strip. The strip isrolled onto the form 148 in a jelly-roll fashion, capturing the strands146 between the various turns 144 of the roll 142 as it forms. As therolling proceeds, all of the strands 146 are captured between the turns144 of the roll 142.

The B-staged material of the roll 142 is cured in the normal fashion.The curing usually involves placing the roll 142 (including the capturedstrands 146 and the form 148) into an autoclave or a pressure bag withina furnace. As the curing proceeds, the pressure on the roll forces thestrands 146 of metal to be pressed into the curing prepreg material. Thefibers of the prepreg material tend to surround and support the metalstrands 146 in the final product. The close contact of the prepregmaterial and the strands ensures an absence of a leak path, and thepreviously mentioned spatial diffusion of the thermal expansiondisplacements avoids the development of leaks during subsequenttemperature excursions during service.

After the plug 140 is cured, it is machined as necessary along itsoutside dimensions to fit within the plate bore 138 as shown. The plug140 is placed within the bore 138 and fixed into place with an adhesivesuch as an ambient-temperature curable epoxy. The seal between the plug140 and the plate 132 is accomplished by standard techniques, becausethe thermal expansion of the cured epoxy of the plug 140 and thefiber-reinforced plastic of the plate 132 are comparable.

The structure of the feedthrough 130 has been constructed. Two sampleswere tested by cycling it 20 times between ambient temperature and 4K.There was no failure of the feedthrough.

Although a particular embodiment of the invention has been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

What is claimed is:
 1. A thermal feedthrough, comprising:afiber-reinforced plastic plate having a first surface and a secondsurface, and further having a threaded plate bore therethrough; athreaded bolt made of a metallic alloy and engaged to the threaded platebore, the bolt having an interior bolt bore therethrough; a firstmetallic thermal conductor extending through the interior bolt bore andsealed thereto; a layer of a first adhesive between the threads of thebolt and the bore of the plate; a first retainer engaged between thebolt and the: plate adjacent to the, first surface of the plate; asecond retainer engaged between the bolt and the plate adjacent to thesecond surface of the plate, the second retainer includinga volume of asecond adhesive contacting the second surface of the plate, and a nutthreadably engaged to the bolt; the first retainer, the second retainer,and the bolt cooperating to place the bolt in tension and the plate incompression; a second metallic thermal conductor affixed to the firstmetallic conductor at a first end thereof; and a third metallic thermalconductor affixed to the first metallic conductor at a second endthereof.
 2. The feedthrough of claim 1, wherein the first adhesive andthe second adhesive are selected from the group consisting of an epoxyand a polyurethane adhesive.
 3. The feedthrough of claim 1, wherein theplate is comprised of glass fibers incorporated into a curable polymericmatrix.
 4. The feedthrough of claim 1, wherein the first metallicconductor is made of a material selected from the group consisting ofcopper, a copper alloy, aluminum, an aluminum alloy, silver, a silveralloy, gold, and a gold alloy.
 5. The feedthrough of claim 1, wherein atleast one of the second metallic thermal conductor and the thirdmetallic thermal conductor is affixed to the first metallic conductor bysoldering.
 6. The feedthrough of claim 1, wherein the bolt is made of acopper-beryllium alloy.
 7. The feedthrough of claim 1, wherein thefeedthrough is made entirely of nonmagnetic materials of constructions.